Interest has developed in using beds of buoyant granules as a substitute for sand filtration beds, particularly for use in filtering water for potable use and for wastewater treatment. With buoyant filtration media, liquid is typically pumped upwardly through a bed of the media, which is held in place by a porous retaining means such as a screen, and the filtered water is collected at the top of the bed, as in an overflow weir disposed around the porous screen. See Smith, British Pat. No. 833,327; Miks, Canadian Pat. No. 816,481 (as well as Czechoslovakian Pat. Nos. 141,652; 128,459 and equivalents thereof); a paper by K. W. Simmers at the 40th Annual Meeting of the International Water Conference, Pittsburgh, Pa., U.S.A., October, 1979; and a paper by J. Rozkydalek and I. Tesarik, 14th International Water Supply Association Congress and Exhibition, Zurich, Switzerland, September 1982.
Conventionally with sand filter beds, as well as with the suggested buoyant filter beds, the bed of filtration media is located in a stationary, stabilized apparatus through which liquid to be clarified is passed. Such apparatus requires a significant construction effort, with a support structure such as a foundation, and a superstructure for holding the filter beds and providing means for liquid to be conveyed into and out of the filter. Operating costs are also significant, since energy is required to pump liquid through the bed for clarification, as well as for reversing liquid flow to clean the filter bed.